There are multiple levels of "lost". For example, Miklos Rozsa's MGM scores are lost (they were thrown away) but we have the recordings so can reconstruct them by ear. Havergal Brian's masterpiece, Prometheus Unbound, is lost yet we have the vocal part with a piano reduction of the orchestra so can probably be reconstructed in an approximation. Maybe Akutagawa's ballet's fall in this grey line. Mahler's student symphonies are lost probably forever because they were stored in Dresden when it was destroyed in WWII.
